Re: Ambiguous type variable

I understood typeclasses to be analogous to OO interfaces. So if a variable implements the Exception interface, and Exception implements the Show interface, then it should automatically support show.

I take it this was wrong? How does the compiler use typeclasses if they're not interfaces?

Francesco Ariis wrote:

> I'm trying to use
> catch (...) (\e -> putStrLn $ show e)
> However, I get an error
> Ambiguous type variable ‘a0’ arising from a use of ‘show’ prevents the constraint ‘(Show a0)’ from being solved.
> This goes away if I change the code to
> catch (...) (\e -> putStrLn $ show (e::IOException))
>
> A couple of things I don't understand here:
> - The signature for catch begins "Exception e", and exception it "class (Typeable e, Show e) => Exception e". So why isn't show automatically available?
> - Why does the new code work at all? e is Exception, not IOException. What would happen if it caught a different Exception?

IOException is a concrete type while Exception is a typeclass. In the end,
the compiler needs the former, the latter not being enough.

The code works as any other class-based function would

someFunction :: Monoid a -> [a] -> a
-- ^-- in the end `Monoid a` will become something concrete, like
-- a String, a Sum, etc.

I understood typeclasses to be analogous to OO interfaces. So if a variable implements the Exception interface, and Exception implements the Show interface, then it should automatically support show.

I take it this was wrong? How does the compiler use typeclasses if they're not interfaces?

Francesco Ariis wrote:

> I'm trying to use
> catch (...) (\e -> putStrLn $ show e)
> However, I get an error
> Ambiguous type variable ‘a0’ arising from a use of ‘show’ prevents the constraint ‘(Show a0)’ from being solved.
> This goes away if I change the code to
> catch (...) (\e -> putStrLn $ show (e::IOException))
>
> A couple of things I don't understand here:
> - The signature for catch begins "Exception e", and exception it "class (Typeable e, Show e) => Exception e". So why isn't show automatically available?
> - Why does the new code work at all? e is Exception, not IOException. What would happen if it caught a different Exception?

IOException is a concrete type while Exception is a typeclass. In the end,
the compiler needs the former, the latter not being enough.

The code works as any other class-based function would

someFunction :: Monoid a -> [a] -> a
-- ^-- in the end `Monoid a` will become something concrete, like
-- a String, a Sum, etc.

Re: Ambiguous type variable

On Thu, Aug 17, 2017 at 12:24:07PM +0000, Jonathon Delgado wrote:
> I'm sure it makes sense! I'm not really following though.
>
> I understood typeclasses to be analogous to OO interfaces. So if a variable
> implements the Exception interface, and Exception implements the Show
> interface, then it should automatically support show.
>
> I take it this was wrong? How does the compiler use typeclasses if they're
> not interfaces?

That's correct! Indeed ghc is not complaining about a lack of
instances, as it would with, say

λ> putStrLn 5
-- • No instance for (Num String) arising from etc etc.

but about the *ambiguity* of type variable `e`. How does `catch` know
_which_ exception to deal with if you don't specify the concrete type?
Consider:

Re: Ambiguous type variable

On Thu, Aug 17, 2017 at 12:24:07PM +0000, Jonathon Delgado wrote:
> I'm sure it makes sense! I'm not really following though.
>
> I understood typeclasses to be analogous to OO interfaces. So if a variable
> implements the Exception interface, and Exception implements the Show
> interface, then it should automatically support show.
>
> I take it this was wrong? How does the compiler use typeclasses if they're
> not interfaces?

That's correct! Indeed ghc is not complaining about a lack of
instances, as it would with, say

λ> putStrLn 5
-- • No instance for (Num String) arising from etc etc.

but about the *ambiguity* of type variable `e`. How does `catch` know
_which_ exception to deal with if you don't specify the concrete type?
Consider:

If I switch ArithException and ErrorCall the behaviour of invoking `palla`
changes.

Having a catch-all `catch` is possible by using (e :: SomeException);
if you don't care about `e` and just want to do an action regardless, you
are probably better off with `onException`.

This is dangerous: `catch` with `e :: SomeException` will catch all asynchronous exceptions, breaking things like timeout, race, and the async library in general. That's why my message about mentioned the safe-exceptions package.